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THURSDAY, April 4, 2013 — Vaccinations are one of the hottest health topics on Twitter these days, but whether you're pro vaccines or against them, how do you measure the impact of your tweets or the power of social media to persuade health-related debates?

In fact, tweeting positively about vaccinations may have the perverse effect of encouraging others to respond negatively, according to a new study from Penn State University. The researchers also found that tweets promoting negative views of vaccinations spread more easily than pro-vaccine messages.

Starting back in 2009 at the height of the H1N1 flu virus outbreak, the Penn researchers used vaccine-related keyword and phrase searches to track reaction patterns by Twitter users to pro and con tweets on vaccinations, in general, as well as on the new vaccine for H1N1. They then had university students rate the intent of 10 percent of the messages as positive, negative, neutral, or irrelevant.

Using a computer algorithm, they analyzed the remaining 90 percent of tweets on flu vaccinations. "The human-rated tweets served as a 'learning set' that we used to 'teach' the computer how to rate the tweets accurately," said study leader Marcel Salathé, PhD, assistant professor of biology at Penn State, in a press release. The final tally of more than 300,000 tweets included samples for all ratings categories.

Researchers then analyzed tweets by rating category, the number of followers for each Twitter handle (which were also rated as having either a negative or positive stance on the H1N1 vaccine), how many tweeters who were followed that returned the favor, and tweet history.

"How many pro-vaccine or anti-vaccine individuals a Twitter user follows is just one measure," Dr. Salathé added in the release. "Also important is how many negative or positive tweets each followed microblogger then broadcasts to his followers. It might be that a Twitter user follows only 5 anti-vaccine microbloggers, but if those 5 microbloggers all send 10 negative tweets per day, that might have an important impact."

The researchers concluded that:

Negative views on vaccinations were more contagious on Twitter than positive views.

The negative opinions of tweeters that people follow fueled even more negative tweeting from the original user. Positive tweeting had no effect.

A larger number of both negative and positive tweets encouraged others to tweet more negatively.

Despite their findings on the reach of social media to negatively influence opinions on vaccinations, the researchers were positive about Twitter's tracking capabilities regarding other potential future health issues. "While some of our results from the H1N1 study may seem frustrating, there are silver linings," Salathé said in the release. "First, we have a tried-and-true way to track and analyze the wealth of data out there on Twitter. Second, further studies may reveal why positive messages seem to encourage negative tweeting; perhaps there's something about the manner in which the message is being conveyed. For example, public health officials could use that information to send positive messages in a way that would be more likely to have the intended effect."